Intrusion detection system



March 14, 1967 c, G. KEENEY INTRUSION DETECTION SYSTEM 2 Sheets-$heet 1Filed 001:. 30, 1964 IE1 To LIGHT SOURCE & m

N Mw L R D 0 LM 6/ W T T D 9 R 5/ E P M A RU A C R A U m 4 O G/W U E R TM T 3 6/ R E cw L P M A INVENTOR.

CLARE G. KEENEY VOLTAGE DIV IDER 2O ATTORNEY United States fPatent()filice Patented Mar. lid, "i967 3,309,689 HNTRUSION DETECHGN SYSTEMIiare G. Keeney, Campbell, Calif assignor to Sylvania Electric ProductsInc, a corporation of Delaware Filed Get. 3t 1964, Ser. No. $17,713 8(Ilaims. (1. 340-4258) This invention relates to intrusion detectionsystems and more particularly to light responsive apparatus fordetecting movement of an intruder in a prescribed area.

Although there are a number of different type intrusion detectionsystems such as radio frequency and sound detection systems, capacitancetype perimeter detection systems, and so-called electric eye or beaminterrupter perimeter detection systems, no effective economical systemis available for protecting residential homes. This invention isdirected toward provision of such a system.

In accordance with this invention, two radiation-sensitive elements,such as photoconductive cells, are physically arranged in a prescribedarea so the normals to the radiation-sensitive surfaces intersect. Aradiation source, such as a light, is located in front of the elements,preferably on the bisector of the angle of intersection. The elementsform a network, such as a voltage divider. The output of the network,which is proportional to the relative change in the response of theelements to incident radiation, is connected to a signal processingcircuit. When an intruder moves between one element and the effectiveradiation source (including the walls which may refiect radiation to theelements), there is a change in the intensity of the radiation incidenton the one element whereas the intensity of the radiation incident onthe other element remains relatively constant. Thus, the response of theone element and the output of the network vary. This variation isprocessed by the signal processing circuit. If the output of theprocessor exceeds a predetermined level, an alarm circuit indicatesintrusion of the protected area. If the intensity of the radiationsource varies, there is equal change in the radiation incident on andthe response of each element. An alarm is not generated by thiscondition, however, since the output of the network remains relativelyconstant. In a modified form of this invention, a thirdradiation-sensitive element is located in the processing circuit. Thisthird element maintains the sensitivity of the apparatus relativelyconstant by making the gain of the processor vary inversely withvariations in the intensity of the ambient radiation environment.

An object of this invention is the provision of economical intrusiondetection system for protecting residential homes.

Another object is the provision of intrusion detection apparatus thatoperates in conjunction with conventional sources of light.

Another object is the provision of intrusion detection apparatus whichhas relatively constant sensitivity and is self-compensating such thatmost changes in the intensity of ambient light will not alarm thesystem.

A further object is the provision of intrusion detection apparatus thatis self-contained.

Another object is the provision of intrusion detection apparatus that isrelatively simple to install and operate.

Still another object is the provision of intrusion detection apparatuswhich is highly portable for convenient relocation in areas to beprotected.

These and other objects of this invention will be more fully understoodfrom the following description of a preferred embodiment thereof,illustrated in the accompanying drawings in which:

FIGURE 1 is an isometric view of portable self-contained intrusiondetection apparatus embodying this invention;

FIGURE 2 is a house, partly in section, illustrating the use ofequipment embodying this invention to prevent undetected intrusion of aroom thereof;

FIGURE 3 is a circuit diagram of a preferred embodient of thisinvention;

FIGURE 4 is a modified form of the embodiment of FIGURE 3 for making theprocessing circuitry thereof responsive to outputs of a plurality ofpairs of photosensitive cells;

FIGURE 5 is a modified form of the embodiment of FIGURE 3 for preventingdefeat thereof by an intruder; and

FIGURE 6 is a schematic plan view, partially in section, of a modifiedform of the invention showing photosensitive cells in a translucentprism.

Specific reference being had to the equipment illustrated in FEGURE 1,the intrusion detection apparatus 1 preferably comprises a box-likeenclosure having a pair of photoconductive cells 2 and 3 mounted inadjacent walls 4 and 5, respectively, of the enclosure. Thephotosensitive surfaces of cells 2 and 3 lie in perpendicular planes,which may be coincident with or parallel to the planes of enclosurewalls 4 and 5, respectively, intersecting at edge E. Although the planesincluding the photosensitive surfaces of cells 2. and 3 intersect toform an angle a of 90, these planes may be oriented to intersect atother angles between 0 and 180. The cells 2 and 3 are electricallyconnected as illustrated in FIGURE 3 to form a voltage divider. A thirdphotoconductive cell 6 is mounted in wall t. A potentiometer 7 providesfor manual adjustment of the sensitivity of the apparatus. A speaker 8mounted in wall 5 provides an audio indication of intrusion of theprotected area.

The room in FIGURE 2 is illuminated by a light source 11, such as aconventional table lamp. The intrusion detection apparatus 1 is locatedopposite the light source between doors 12 and 13. The light source 11is preferably located on the bisector B of the 90 angle a, see FIGURE 1,formed by the intersection of the planes containing the photosensitivesurfaces of cells 2 and 3 to provide approximately equal illumination ofeach cell. Light rom source 11 is transmitted to intrusion detectionapparatus 1 directly (path 14) and by reflection from walls (path 15)and objects in the room (path 16). The cells have maximum response toincident light that is normal to the plane of the photosensitivesurfaces thereof.

When objects in the room are stationary, the resistance ratio of thevoltage divider network comprising cells 2 and 3 is constant andintrusion of the room is not indicated. When an intruder moves about theroom, however, he shadows the cells by cutting light paths such as theone indicated at 15. As the intruder moves across the light path 15, thevariation of the intensity of light incident on cell 2 is much greaterthan the corresponding variation in light incident on cell 3. Thus, theresistance of cell 2 changes with respect to the resistance of cell 3and the resistance ratio of the voltage divider varies. The output ofthe voltage divider is processed and intrusion of the room indicated ifit exceeds a predetermined level. The intrusion detection apparatus 1illustrated in schematic form in FIGURE 3 comprises a voltage divider 20having photoconductive cells 2 and 3 electrically connected in seriesbetween the negative supply otential V (such as a battery) and a groundreference potential. The voltage divider may be considered to be asignal generator or voltage source and resistance connected across theinput of buffer amplifier 23. The output of the voltage divider on line21 is applied to an AC. amplifier 22 comprising amplifier stages 23through 27, inclusive. Amplifier stages 23 and 27 are preferably bufferstages which isolate the associated amplifier stages 24 and 26 from thevoltage divider and output circuitry, respectively.

49 Amplifier stage is preferably a buffer amplifier which isolates theamplifier stages 24 and 26 from each other and provides means foradjusting the sensitivity of the apparatus. The buffer amplifiers may,by way of example, be emitter followers.

The sensitivity of the apparatus is controlled by a resistive biasingnetwork in the emitter circuit of buffer amplifier 25. The biasingnetwork is a voltage divider comprising potentiometer 7 andphotoconductive cell 6. Potentiometer '7 is electrically connected inseries between the emitter of transistor 25' and ground. The control armof the potentiometer is connected through cell 6 to ground. Thesensitivity is adjusted automatically by cell 6 to compensate forvariations in the ambient light intensity in the protected area. Thesensitivity is adjusted manually by potentiometer 7.

The output of A.C. amplifier 22 is applied on line 28 to a clamper 31comprising capacitor 32, resistor 33 and diode 34. The clamped signal isdetected by an amplitude detector 35 comprising diode 36, capacitor 37and resistor 33.

The detected signal is applied on line 40 to a threshold circuitcomprising a monostable multivibrator 41. The output of multivibrator 41is applied on line 44 to the base of a transistor 45 of switch 45.Transistor 45 is electrically connected across the input circuit oftransistor 46 of driver 46. An astable multivibrator 47, whichoscillates at an audio rate, is also connected across the input ofdriver 46.

The output of driver 46 is electrically connected through a couplingtransformer 48 to the negative supply potential V. The output of thedriver is also coupled through the transformer to a speaker 49 forproviding an audio indication of intrusion of the protected area.

When objects in the protected area are stationary, the intensity oflight incident on the photosensitive surfaces of cells 2 and 3 isrelatively constant. The resistance ratio determining the output of thevoltage divider is therefore constant and is represented as s where R isthe resistance of cell 2 and R is the resistance of cell 3.

The magnitude of the output of A.C. amplifier 22 and the signal on line49 are relatively low (the latter being a small negative D.C. signal).This signal on line 4d reverse biases and cuts off transistor 42 ofmultivibrator 41. The signal on line 44, therefore, is a negativevoltage that forward biases transistor 45 and drives it into saturation.Conduction of transistor 45' short circuits the input of driver 4-6,cutting off the latter. Thus, the audio output of multivibrator 47 isblocked from speaker 49 and intrusion of the protected area is notindicated.

If an intruder moves about in the protected area, he will shadow one ofthe photoconductive cells and different amounts of light will beincident on cells 2 and 3. Consider that movement of an intruder causesthe light incident on cell 2 to decrease. This causes the resistance Rof the cell to increase. The resultant resistance ratio of the voltagedivider is represented as 2+ zl 2+ 2) s where AR represents the changein the resistance of cell 2. This change causes the voltage divideroutput on line 21 to become more negative, i.e., a negative goingsignal, that is amplified.

If the intruder generates a repetitive varying signal, the amplifiedsignal on line 28 is effectively clamped to ground and is doubled byoperation of clamper 31. The clamped signal is detected to provide aD.C. signal proportional to the peak magnitude thereof. If this DC.signal is more negative than a predetermined level, transistor Cal 42 isforward-biased and conducts. The output of transistor 42 is a very smallnegative potential (approximately 0.3 volt).

Transistor 55, reverse-biased and cut off by the output of transistor42, is effectively an open circuit across the input of driver 46 and thelatter conducts. The audio output of multivibrator 47 is amplified bythe driver and is coupled to speaker 49 to provide an audio indicationof intrusion of the protected area.

It is desirable that changes in intensity of the light source do notcause the detection apparatus to indicate intrusion of the protectedarea. This is accomplished by employing photoconductive cells 2 and 3 aselements of voltage divider 29.

A change in the intensity of the light source 11 (see FIGURE '2) isreflected equally on cells 2 and 3, since the light source is positionedto provide approximately equal illumination of each cell. This change inlight intensity causes an equal change in the resistance of each cell.The resultant resistance ratio of voltage divider 29 is represented asR2+ z 2+ 2)+( 3+ s) where AR; and AR;, represent the change inresistance of cells 2 and 3, respectively. This resistance ratio and theoutput of the voltage divider are relatively constant for such changesin light intensity since the cell resistance R is equal to the cellresistance R and the resistance change AR is equal to the resistancechange AR (assuming the response curves of the cells are identical).Thus, a change in the intensity of light source 11 does not causedetection apparatus 1 to indicate intrusion of the protected area,unless the light source is abruptly extinguished.

Intrusion detection apparatus 1 may be employed in a home to protect adetached building, such as a garage, over a time period from sunsetuntil after sunrise. During such an operating period, the apparatus issubject to a wide variation of ambient light intensity. It is desirablein such applications that system sensitivity to intruder movementsremain relatively constant as the intensity of the light source varies.

When an intruder moves in the protected area and shadows only one cell,there is a change in the resistance of the shadowed cell, the resistanceratio (see Equation 1) and the output of the voltage divider. Theresistance of the cell is represented as ln(R):Kln(I)+ C (4) in which lnis the natural logarithm, R is the cell resistance, K and K areconstants and I is light intensity. The rate of change of the resistanceof the cell, with respect to the rate of change of light intensity, isproportional to the ratio of the cell resistance and light intensity andis represented as where d is the derivative.

The absolute value of the output V of the voltage The rate of change ofthe output V of the voltage divider, with respect to the resistance ofcell 2, is obtained by differentiating Equation 6, and is represented aslaw) l iRi) l 1) The rate of change of the output V of the voltagedivider, as a function of the rate of change of the light intensity isobtained from Equations 5 and 7 (where R:R =R and is represented as 1(8) Thus, the rate of change of the output V of the voltage divi er(which is proportional to the system sensitivity) is proportional to theratio of the rate of change of the light intensity to the absolute lightintensity. This ratio, d(I)/1, represents the percentage change in thetensity of light incident on the shadowed cell. This percentage changein light intensity, for a given movement within the protected area, isthe same whether the effective light intensity is bright or dim, sincethe shadowing of the cell is the same in each case. Thus, thesensitivity of the intrusion detection apparatus is constant fordifferent intensities of the light source.

In an actual embodiment and application of intrusion detection apparatus1, the system sensitivity is not constant; since ditferentphotoconductive cells do not have identical responses, the cells areloaded by external circuitry and the apparatus may not be adjusted soequal amounts of light are incident on each cell. The thirdphotoconductive cell 6 is employed in this invention to automaticallyadjust the sensitivity of apparatus 1 to make the system sensitivitymore nearly constant for changes in intensity of the light source.

Cell 6 is preferably located so its photosensitive surface faces thesame direction as one of the cells 2 and 3 (see FIGURE 1). The cell 6 iselectrically connected to change the gain of A.C. amplifier 22 (see FIG-URE 2).

The resistance of cells 2 and 3 decrease as the intensity of the lightsource increases. This decrease in cell resistance decreases theinternal impedance of the voltage divider (signal generator) and thesystem sensitivity therefore increases.

The resistance of cell 6 also decreases as the intensity of the lightsource increases. This resistance change decreases the gain of bufferamplifier 25 and less of the AC. signal is applied to transistor 26'.This decrease in amplifier gain caused by cell 6 tends to cancel theincrease in amplifier gain and system sensitivity caused by theimpedance change of the voltage divider and the system sensitivitytherefore remains relatively constant.

Conversely, the resistance of cells 2, 3 and 6 increase as the intensityof the light source decreases. This change in resistance of cells 2 and3 increases the internal impedance of the voltage divider (signalgenerator) and the system sensitivity therefore decreases. The increasein the resistance of cell'6, however, increases the gain of bufferamplifier 25 and more of the AC. signal is applied to transistor 26.Thus, the increase in gain caused by cell 6 cancels the decrease in gainand system sensitivity caused by the voltage divider and the systemsensitivity remains relatively constant. The system sensitivity ismanually adjusted by varying potentiometer 7.

In certain applications, it may be necessary to employ a plurality ofpairs of photoconductive cells 2 and 3 to protect a large or irregularlyshaped area or to simultaneously protect a number of physically separateareas. In such applications, it may not be desir able or economicallyfeasible to employ more than one of the self-contained intrusiondetection apparatus illustrated in FIGURE l. A modified form of thisinvention in which a plurality of cell pairs are connected to commonprocessing circuitry in one detection apparatus is illustrated in FIGURE4.

Voltage dividers 20a, 29b, and 28c comprise cell pairs 2a3a, 212-312 and2c3c, each electrically connected between the negative supply potentialV and the ground reference potential. The cell pairs may be located inphysically separate areas such as kitchen, bedroom and detached garage.The outputs of the voltage dividers are applied on associated lines 51a,51b and 51c to a summing network comprising resistors 52a, 52b, 52c and53. The sum of the respective outputs of the cell pairs is applied online 2 to the base of transistor 23' of AC. amplifier 22 (see FIGURE 3).

In certain instances, intruders may deliberately attempt to defeat theintrusion detection apparatus. The embodiment shown in FIGURE 5comprises means for preventing an intruder who has gained access to thelight source from defeating the system by extinguishing the ight orincreasing the intensity of the light source to such a degree as tosaturate the cells and thus minimize or eliminate their response tochanges in ambient light. The voltage divider 2%? comprises cell 3'connected through res tor 55 to the negative supply potential and cell 2connected through resistor 57 to the ground ref rence potential. Thevoltage divider output on line 58 is applied to processing circuitrycomprising a 11C. amplifier 59', a threshold detector 60 and an alarmcircuit 61. The voltage divider output on line 62 is applied to similarprocessing circuitry comprising amplifier 63, detector 64 and alarmcircuit 65. Detectors 60 and as may, by way of example, comprise Schmitttrigger circuits. The voltage divider output on line 21 is applied tothe base of transistor 23' of AC. amplifier 22 (see FIGURE 3).

The resistance of cells 2. and 3 increase as the intensity of incidentlight decreases, such as when the light source is gradually turned off.This resistance change causes the potential on line 58 to decrease(become more negative). When the potential on line 58 exceeds (is morenegative than) a predetermined level, threshold detector 60 conducts toindicate tampering or failure of the light source.

Similarly, the resistance of cells 2 and 3' decrease as the intensity ofincident light increases, such as when the intensity of the light sourceis increased. This decrease in resistance causes the potential on line52 to decrease (become more negative). When the potential on line 62, ismore negative than a predetermined level, threshold detector 64 conductsand tampering with or failure of the light source is indicated.

Another embodiment of the invention shown in FIG- URE 6 features meansfor protectively covering the active surfaces of the photocells and forpreventing false alarms due to insects such as flies moving on thosesurfaces. A translucent prism 79, such as a cubical body of glass, hasphotoconductive cells 71 and 72 mounted adjacent to prism surfaces 73and 74, respectively. The photosensitive surfaces '71:: and 72a of thecells face into the prism body 7% so that rays r and 7- from the li htsource, not shown, pass through the body before reaching the active cellsurfaces. The prism dimensions a between cell surfaces 71a and 72a andthe opposite prism surfaces and 7011, respectively, preferably aresulficient to adequately space insects from surfaces 71:1 and 72a andthus minimize deleterious shadowing effects on the cells from thesesources. The maximum responses of cells 71 and 72 occur for light raysthat are normal to surfaces 71a and 720, as for the previously describedembodiments. Cells 71 and 72 are electrically connected to processingcircuits described in conjunction with FIG- URE 3 to indicate intrusion.

Although this invention has been shown and described in relation topreferred embodiments thereof, variations and modifications will beapparent to those skilled in the art. The scope and breadth of thisinvention is therefore to be determined from the following claims ratherthan from the above detailed description.

What is claimed is:

1. A system for detecting intrusion of a room comprising an electriclamp at a first location in said room,

detection apparatus at a second location in said room,

said detection apparatus comprising a body having a plurality of planesides with the planes of the sides intersecting each other,

said body being oriented with the intersection of the planes of two ofsaid sides closer to said lamp than the remainder of the body,

a light responsive element mounted on the exterior of each of said twosides and substantially equally spaced from said lamp and at a greaterdistance therefrom than said intersection of the planes of said twosides,

each of said elements being characterized by an electrical resistancethat is a function of the intensity of light incident thereupon,

a source of electrical energy connected to said elements,

said elements being electrically connected in series to form a voltagedivider network,

a signal processing circuit connected to said network and beingresponsive to a relative change in voltages across said elements toproduce an output, and

an alarm circuit responsive to said output of the signal processingcircuit to produce an alarm.

2. A system according to claim 1 with a plurality of pairs of saidelements at dilferent locations,

the elements of each pair being oriented relative to each other so thatnormals from their surfaces diverge,

said signal processing circuit being responsive to each of the outputsof said pairs of elements whereby an alarm is produced by an intrusionat any of said locations.

3. A system for detecting intrusion of a room comprising an electriclamp at a first location in said room,

detection apparatus at a second location in said room.

said detection apparatus comprising a body having a plurality of planesides with the planes of the sides intersecting each other,

said body being oriented relative to the lamp so that the planes of twoof said sides converge in a direction toward said lamp,

a light-responsive element mounted on the exterior of each of said twosides and substantially equally spaced from said lamp and at a great-erdistance therefrom than the inter section of the planes of said twosides,

each of said elements being characterized by an electrical resistancethat is a function of the intensity of light incident thereupon,

"a source of electrical energy connected to said elements,

said elements being electrically connected in series form a voltagedivider network,

a signal processing circuit connected to said network and responsive toa relative change in voltages across said elements to produce an output,

an alarm circuit responsive to said output of the signal processingcircuit to produce an alarm, and

a third light-responsive element mounted on the exterior of said bodyand connected to said signal processing circuit for changing thesensitivity of the signal processing circuit in inverse proportion tothe change in absolute level of light intensity in the room.

4. A system for detecting intrusion of a room comprising :an electriclamp at a first location in said room, and

detection apparatus comprising 7 a body at a second location in saidroom having a plurality of plane sides with the planes of the sidesintersecting each other,

said body being oriented relative to the lamp so that the planes of twosaid sides converge in a direction toward said lamp,

a light-responsive element mounted on the exterior of each of said twosides and substantially equally spaced from said lamp and at a greaterdistance therefrom than the intersection of the planes of said twosides,

each of said elements being characterized by an electrical resistancethat is a function f the intensity of light incident thereupon,

a source of electrical energy connected to said elements,

said elements being electrically connected in series to form a voltagedivider network,

a signal processing circuit connected to said network and responsive toa relative change in voltages across said elements to produce an output,

an alarm circuit responsive to said output of the signal processingcircuit to produce an alarm,

circuit means responsive to the total change in voltage across saidelements producing an output when said total voltage reaches a limit,and

a second alarm circuit connected to said circuit means and responsive 0tan output therefrom for producing an alarm.

5. A system for detecting intrusion of a room comprising in combinationa source of light at a first location in said room,

and detection apparatus at a second location in said room,

comprising a body having at least two sides lying in intersectingplanes,

a light-responsive element having a photosensitive surface and mountedon each of said sides whereby to change a condition in response to achange in the intensity of light incident on said photosensitivesurface,

said light source being spaced farther from said elements than from theintersection of said planes whereby said photosensitive surfaces aredivergent relative to each other,

means for comparing said conditions controlled by said elements forderiving an output when one condition changes relative to the other,

signal processing means responsive to said relative change of conditionsfor indicating an alarm and a third light-responsive element on saidbody elec trically connected to said signal processing means forchanging the sensitivity thereof inversely with the change in absolutelevel of intensity of light in said room.

6. A system for detecting intrusion of a room comprising in combinationa source of light at a first location in said room,

and detection apparatus at a second location in said room comprising abody having at least two sides lying in intersecting planes,

a light-responsive element having a photosensitive surface and mountedon each of said sides whereby to change a condition in response to achange in the intensity of light incident on said photo-sensitivesurface,

said light source being spaced farther from said elements than from theintersection of said planes whereby said photosensitive surfaces aredivergent relative to each other, means for comparing said conditionscontrolled by said elements for deriving an output when one conditionchanges relative to the other, signal processing means responsive tosaid rela tive change of conditions for indicating an alarm, circuitmeans responsive to the total change of intensity of light in said roomand producing an output when said total change of light intensityreaches a limit, and a second alarm circuit connected to said circuitmeans and responsive to said output therefrom to produce an alarm. 7. Asystem for detecting intrusion of a room comprising in combination asource of light at a first location in said room,

and portable detection apparatus comprising a body at a second locationin said room and having at least two external sides lying inintersecting planes so that normals from said sides diverge therefrom,light-responsive elements having maximum response to light receivedalong the said normals being mounted on said body sides, respectively,each of said elements being adapted to change a condition in response toa change in the intensity of the light received by the element,

of con- References Cited by the Examiner UNITED STATES PATENTS 1,877,2799/1932 Dawson 340-258 2,016,036 10/1935 Fitzgerald 340228 2,227,14712/1940 Lindsay. 2,656,527 10/1953 Tillman. 3,089,065 5/1963 Worden.3,191,048 6/1965 Cowen 340228 X 3,221,317 11/1965 Ferrigno 3403S4 X NEILC. READ, Primary Examiner.

THOMAS E. HABECKER, Examiner.

R. GOLDMAN, D. L. TRAFTON, Assistant Examiners.

1. A SYSTEM FOR DETECTING INTRUSION OF A ROOM COMPRISING AN ELECTRICLAMP AT A FIRST LOCATION IN SAID ROOM, DETECTION APPARATUS AT A SECONDLOCATION IN SAID ROOM, SAID DETECTION APPARATUS COMPRISING A BODY HAVINGA PLURALITY OF PLANE SIDES WITH THE PLANES OF THE SIDES INTERSECTINGEACH OTHER, SAID BODY BEING ORIENTED WITH THE INTERSECTION OF THE PLANESOF TWO OF SAID SIDES CLOSER TO SAID LAMP THAN THE REMAINDER OF THE BODY,A LIGHT RESPONSIVE ELEMENT MOUNTED ON THE EXTERIOR OF EACH OF SAID TWOSIDES AND SUBSTANTIALLY EQUALLY SPACED FROM SAID LAMP AND AT A GREATERDISTANCE THEREFROM THAN SAID INTERSECTION OF THE PLANES OF SAID TWOSIDES, EACH OF SAID ELEMENTS BEING CHARACTERIZED BY AN ELECTRICALRESISTANCE THAT IS A FUNCTION OF THE INTENSITY OF LIGHT INCIDENTTHEREUPON, A SOURCE OF ELECTRICAL ENERGY CONNECTED TO SAID ELEMENTS,SAID ELEMENTS BEING ELECTRICALLY CONNECTED IN SERIES TO FORM A VOLTAGEDIVIDER NETWORK, A SIGNAL PROCESSING CIRCUIT CONNECTED TO SAID NETWORKAND BEING RESPONSIVE TO A RELATIVE CHANGE IN VOLTAGES ACROSS SAIDELEMENTS TO PRODUCE AN OUTPUT, AND AN ALARM CIRCUIT RESPONSIVE TO SAIDOUTPUT OF THE SIGNAL PROCESSING CIRCUIT TO PRODUCE AN ALARM.